This invention relates to water-cooled nuclear reactors; and, more specifically, to a water-cooled, water-moderated nuclear reactor having a passive emergency shutdown and core cooling capability.
Nuclear reactors are the principal means for converting the large amounts of energy released by nuclear fission into useful thermal energy. When a fissionable atom of uranium or plutonium isotope such as U.sup.233, U.sup.235, and Pu.sup.239 absorbs a thermal neutron, there is a high probability that it will undergo nuclear fission, splitting into two fission products of lower atomic weight having great kinetic energy and emitting a number of neutrons. In a nuclear reactor, the kinetic energy of the fission products is dissipated as heat in the nuclear fuel elements and is removed from the reactor by a coolant in heat exchange relationship with the fuel elements. The fission neutrons are slowed down to thermal range by a moderator and, in turn, used to induce a subsequent fission in another atom in order to keep the reaction self-sustaining.
Water-cooled nuclear reactors possess a number of advantages which make them especially attractive for use in power generation. In these reactors, water performs the dual function of cooling the reactor core and moderating fission neutrons. Water-cooled reactors of many types are described in the literature of the art. (See, for example, J. K. Pickard, ed., Nuclear Power Reactors, Van Nostrand, 1957.) A typical heterogeneous water-cooled nuclear reactor comprises, in essence, a reactor pressure vessel and a nuclear chain reacting core made up of a plurality of nuclear fuel element assemblies. Each fuel assembly comprises an open-ended tubular flow channel surrounding a bundle of rod-type nuclear fuel elements--each of which is typically zirconium-clad enriched uranium oxide. Water is circulated through the channels and around the fuel rods both to remove heat and to act as a moderator.
One of the problems facing the designers of water-cooled reactors is that of providing adequate emergency shutdown and core cooling capability. Because nuclear reactor cores are capable of spontaneous energy generation, and because of the critical necessity for containing within appropriate shielding nuclear fuels and the reaction products thereof, emergency shutdown and core-cooling systems are essential. Systems must be provided which can, in the event of an emergency such as a failure of the coolant pumping system: (1) stop the self-sustaining chain reaction; and (2) dissipate the residual heat (a) stored in the reactor core, and (b) generated by spontaneous decay after shutdown. Such systems are required in order to ensure that, in the event of such an emergency, the reactor core cannot accumulate sufficient heat to penetrate or destroy its shielding.
A major shortcoming of conventional reactor designs is that they typically rely upon active hardware to perform emergency shutdown or emergency cooling. For example, reactor cores are typically located some distance from any heat sink having a capacity adequate for emergency core cooling. As a consequence, they require prolonged and sustained operation of active hardware, such as water pumps, to effect the necessary transfer of residual core heat. Such systems are potentially vulnerable to conceivable accidents and failures because they are critically dependent upon sources of energy external of the core and upon means for delivering that energy.